Normally, an optical fiber cable includes a fiber which consists of a core and a surrounding cladding. To facilitate handling, the cladding should be surrounded by a protective covering. The radiation is conduct ed in the core, whose refractive index differs from that of the cladding. In a step index optical fiber, total internal reflection in the interface is achieved as long as the angle of incidence of the radiation does not exceed a critical angle referred to as the numerical aperture of the fiber. In a graded index optical fiber, the refractive index of the core is distributed, mostly as a parabolic function, and the numerical aperture depends on where the ray impinges upon the end surface. The highest numerical aperture is attained at the centre of the fiber, and this value is indicated in the fiber specification. In all cases, the cladding should be substantially free from radiation. Normally, the refractive index of the core is higher than that of the cladding, and the numerical aperture is determined by this difference in refractive index.
In the transmission of high-power laser light, radiation in the cladding may cause serious problems, since it is eventually absorbed in the covering, resulting in considerable heat generation and damage to the fiber cable. Radiation in the cladding may, for instance, arise as a result of light scattering in the end surface of the fiber, for example caused by scratches or dust particles, or as a result of some of the incident radiation quite simply falling outside the numerical aperture, of the fiber. These problems are most severe at the inlet and outlet ends of the fiber cable, where the reflection of incoming and going radiation, as well as the absorption of dust particles and the like, gives rise to harmful heat generation. At the outlet end, radiation can be reflected from the workpiece directly into the cladding or fall completely outside the fiber. In both cases, the radiation is finally absorbed in the covering and heat is generated.
U.S. Pat. No. 4,678,273 (corresponding to SE Patent 443,454) discloses an optical fiber surrounded by a radiation-proof transmitting material which conducts radiation away from the cladding.
U.S. Pat. No. 3,843,865 discloses an optical fiber having a core without a cladding, the diameter of the core increasing towards the end surfaces. The problem of radiation that is not linked into the core is not touched upon. Also, the fiber is apparently not suited for powers above 200 W, which is not enough for modern materials-processing techniques using laser.